The invention relates to the use of polyimide and a lithographic method for producing microcomponents with component structures in the sub-millimeter range, in which a structurable adhesive layer is applied to a metal layer, and a layer of photostructurable epoxy resin is applied to the adhesive layer. The epoxy resin is structured by selective exposure and removal of the unexposed areas. After removal of the adhesive layer from the gaps between the resin structures, the gaps are filled with metal by electrodeposition. The invention further relates to a method for producing a composite material comprising a substrate, metal, and photostructurable epoxy resins.
In lithographic processes various polymers are used as resist materials. Resist materials are defined as materials that can be structured by means of exposure to light.
PMMA is the most widely used resist material but has the drawback that synchrotron radiation must be used for exposure to produce microstructures with an aspect ratio  greater than 10. This is both time-consuming and costly.
Attempts have therefore been made to switch to photostructurable resist materials that can be structured, for instance, by means of UV light. These materials have the drawback, however, that they cannot be used to obtain large aspect ratios.
A resist material that permits large aspect ratios of, for instance, 15 and above and that can be structured by means of UV light is epoxy resin, particularly an epoxy derivative of a bis-phenol-A Novolac, which is already used in semiconductor technology. This resist material is used in the form of SU-8 resist (trade name of Shell Chemical) and is described, for instance, in J. Micromechanics, Microengineering 7 (1997) pp. 121-124. Large aspect ratios can be obtained because cross-linking as a result of exposure to light causes the refraction index of this material to change, so that structures with waveguide properties can be produced from the resist material. With the aid of masks, vertical walls are obtained by means of light exposure, which are preserved when unexposed areas are etched away.
SU-8-material has the drawback, however, that it does not adhere to all metals or silicon, which are usually used as the starting layers for electrodeposition processes or as substrates.
While SU-8 adheres well to aluminum, its adhesion to gold or nickel depends on the size of the microstructure, i.e. on the lateral dimensions of the microcomponent.
Adhesion to copper, silver, chromium and nickel is less good, so that an adhesive layer is required between the metal and the SU-8 resist.
Proc. SPIE Vol. 3680B-65 Paris, France, March 30 to Apr. 1, 1999, xe2x80x9cMicromachining and Microfabrication,xe2x80x9d entitled xe2x80x9cDesign and realization of a penny-shaped micromotorxe2x80x9d by M. Nienhaus et al. describes the use of a bonding agent, e.g. hexamethyldisilazane (HMDS) between the copper starting layer and the SU-8 material. This has the drawback, however, that the bonding agent is particularly thin, so that adhesion is not satisfactory in all cases.
One object of the invention is thus to provide an adhesive layer that is suitable for photostructurable epoxy resins, particularly for SU-8 resist, and that prevents detachment of the resist. Another object of the invention is to provide a method for producing a composite material and a lithographic method for producing microcomponents, in which adhesion problems related to the resist do not occur.
Surprisingly it has been found that polyimide or polyimide mixtures are excellently suited as the adhesive layer between photostructurable epoxy resin and metals or silicon.
Good adhesion can be obtained on microcomponents with lateral dimensions in the mm and cm range.
Polyimides or photostructurable polyimides that may be considered are described in TRIP. Vol. 3, No. 8, August 1995, pp. 262-271, entitled xe2x80x9cThe Synthesis of Soluble Polyimidesxe2x80x9d by Samual J. Huang and Andrea E. Hoyt as well as in SPIE Vol. 1925, pp. 507-515 entitled xe2x80x9cBase-Catalyzed Photosensitive Polyimidexe2x80x9d by Dennis R. McKean et al. Mixtures of these polyimides are also suitable as adhesive layers.
The method for producing a composite material comprising a substrate, metal and photostructurable epoxy resins is characterized in that a metal layer with microtopography is deposited on the substrate and a polyimide layer is applied to the metal layer as an adhesive layer, to which the epoxy resin is subsequently applied.
Microtopography is defined as roughnesses in the nanometer range. The improvement in the adhesion is achieved by compensating tensions of the polyimide layer in the rough surface.
The polyimide layer is preferably applied with a thickness of  less than 1 xcexcm, preferably with a thickness of 500 to 900 nm. It has been shown that at these small thicknesses the tensions to which the polyimide layer is subject are negligible.
The substrate can be made, for instance, of silicon, glass, plastic, or ceramic, while the metal layer or layers can be a titanium, copper, nickel, and/or silver layer. For instance, if two materials are applied, a titanium layer is applied first and then the copper layer. The layer thicknesses of these metal layers are preferably between 100 and 500 nm. The metal layers can be deposited by means of sputtering processes or vapor deposition.
The metal layer, prior to depositing the polyimide material, is preferably dehydrogenated at 200xc2x0 C. to 300xc2x0 C. for a period of 10 to 60 min. Preferred values are 240xc2x0 C. to 260xc2x0 C. and 25 to 35 min.
The polyimide layer is preferably applied to the dehydrogenated metal layer by means of a spin coat process. After dehydrogenation, no further process steps are required prior to applying the polyimide layer.
Preferably, a precursor material is applied to the metal layer, which is subsequently subjected to a heat treatment to form the polyimide. The precursor materials used are monomer materials of preferably polyamide carboxylic acids. A subsequent heat treatment is used to affect cyclization or ring synthesis, so that polyimide is produced.
The heat treatment is preferably carried out for 0.5 to 2 minutes at 80xc2x0 C. to 100xc2x0 C. and 2 to 4 minutes at 100xc2x0 C. to 120xc2x0 C.
Preferably, UV light is used for floodlight exposure to start the cross-linking process. An additional heat treatment, preferably at 100xc2x0 C. to 110xc2x0 C., serves for further cross-linking. The unexposed or non-crosslinked areas are removed by subsequent development, e.g. with butyl acetate.
The lithographic process for producing microcomponents provides for the use of an adhesive layer of polyimide or a mixture of polyimides, possibly with the addition of bonding agents or photoinitiators. Since the metal layer, to which the epoxy resin is applied over the adhesive layer, is also the starting layer for the subsequent electrodeposition process, those areas where metal is to be deposited must be uncovered. There are two preferred embodiments to accomplish this.
According to the first preferred embodiment, after structuring the epoxy resin, the uncovered zones of the adhesive layer are removed by plasma etching to expose the metal starting layer.
According to the second embodiment, the polyimide used is a photostructurable polyimide.
Other preferred process steps provide for the selective exposure of the adhesive layer of photostructurable polyimide prior to applying the epoxy resin and the removal of the unexposed areas. Subsequently, the epoxy resin is applied all over the adhesive layer, and essentially those areas of the epoxy resin under which the adhesive layer is located are exposed. Thereafter, the unexposed areas of the epoxy resin are removed to uncover the metal layer.
This presumes that both the adhesive layer and the resist layer of epoxy resin are exposed in the same locations.
To this end, preferably the same mask is used for the two exposure processes.
It is also possible to use so-called laser direct-writers whose laser beam is guided over the object to be exposed. When such laser direct-writers are used, the adhesive layer and the epoxy resin layer are each exposed with the same line guidance of the laser beam.